It's not very often that you crack open a book to find a giant warning label hitting you in the face. But there it is right in the front of Scott Carney's latest work, "What Doesn't Kill Us." It says no one should attempt any of these methods or practices without appropriate experience, training, doctor approval, etc. In other words, don't try this at home, kids. The this is exposing yourself to extreme cold. We won't, but we have brought Scott Carney into the studios of Colorado Public Radio in Denver to tell us about his investigation.

Hey, Scott Carney.

SCOTT CARNEY: Hi, how you doing? Thanks for having me on.

GARCIA-NAVARRO: So this is some pretty extreme stuff. The book starts with you climbing a freezing Mount Kilimanjaro practically naked. Please explain the concept.

CARNEY: Well, the idea is that humans - we evolved 200,000 years ago. Our species is basically biologically identical as that caveman way in the past. And only for the last about 150 years have we had control over our environment, where we can live at a constant 72 degrees no matter what the weather is outside.

But our underlying biology is designed to have natural fluctuations, both between night and day and also seasonally. And without those natural fluctuations, we're not able to exist in extreme environments. So I took it sort of upon myself, with the help of a Dutch sort of fitness guru named Wim Hof, to expose myself to extreme environments for about six months to try to create resistance.

GARCIA-NAVARRO: So let me get this straight. The basic idea is that somehow our controlled environments - the air conditioning, the heating, the fact that we are wearing puffy jackets to keep us warm in the winter - all this technology is somehow debilitating us instead of protecting us, which is what most of us think.

You actually started out this whole excursion when you went to go meet this man called Wim Hof, trying to debunk what he was doing. Tell us a little bit about that.

CARNEY: Sure. Well, you know, it started back in 2012 when I'd heard about him. You know, he was this guy who did these crazy feats on mountains. He'd climbed something like two-thirds or three-fourths of the way Mount Everest in just shorts. And not only that, he made these claims he could control his immune system and these things that just sounded, you know, crazy and superhuman. So I was very skeptical.

So I flew out to Poland, and I did the training. And he says, you know, you're going to go stand out in the snow. And I stand in the snow, and in five minutes, I am just done. It's just so cold. I'm in my shorts, bare feet, and it's really, really painful. And so the next day, I did it, and I was able to stand in the snow for 10 minutes. And by the fifth day, I was standing there for an hour.

You know, the basics of the method are, you know, you can activate things like vasoconstriction, which are all of these muscles in your veins through your body which contract when they interact with cold. But you have no conscious way to make those muscles contract. You have to get cold to do that. And if you never get cold, those muscles get weak. And so by reintroducing and intentionally altering your environment, you can really do cool things.

GARCIA-NAVARRO: But not only cool things, you maintain that it can make you less sick. You offer up people suffering from Parkinson's disease and rheumatoid arthritis who say they were helped by this training.

CARNEY: Absolutely. And I have several case studies in the book. And I think we need to make the caveat that this is not rigorously validated by the halls of medicine, although there is certainly ongoing scientific research on it. But I met people with things like Crohn's disease.

And what they were doing was taking cold showers, doing this breathing and to put it, you know, very sort of simply, it was giving their immune system something else to do than attack itself. I met a person whose rheumatoid arthritis, which was keeping him, like, basically stuck in bed, you know, able to walk and do all sorts of things now.

GARCIA-NAVARRO: OK. But if extreme conditioning is so successful, as you report, why hasn't it been more widely accepted?

CARNEY: Well, it is being accepted. I mean, Wim Hof is - the research and the literature around him is certainly growing, but it certainly needs more research.

GARCIA-NAVARRO: I'm sitting here in an climate-controlled studio. And, you know, I want to know - for someone who's not going to climb Mount Kilimanjaro naked - not today, probably not this week, maybe not this month (laughter) - what are the sort of prescriptions that you would give to live a healthier life with what you discovered?

CARNEY: Well, one thing is that you don't need your thermostat up all the way all the time. Like, you can adapt - if you keep your thermostat to 63 or so, right, a point where shivering - you know, you might feel like you might need to shiver - that's probably good. Like, shivering is a natural response.

And one of the things we - you know, we do in the training is we suppress your natural shiver response to make your body find a different way to heat itself. And this is usually ramping up the metabolism. And by doing that, you'll burn more calories, you'll get thinner, but you'll also just be more adapted. You will use less energy in general, and that's a good thing.

GARCIA-NAVARRO: Scott Carney - his new book is called "What Doesn't Kill Us."

Thank you.

CARNEY: Thank you so much for having me on. This has been awesome. Transcript provided by NPR, Copyright NPR.